In electronic devices such as mobile phones, smartphones, and digital video cameras, audio components are mounted in their housings. Such a housing has an opening for allowing sound to pass through. In order to prevent water from entering the housing through the opening, the opening is covered with a waterproof sound-transmitting membrane that allows sound to pass through but prevents water from passing through. As such waterproof sound-transmitting membranes, porous polytetrafluoroethylene (PTFE) membranes are usually used.
Patent Literature 1 discloses a waterproof sound-transmitting membrane having both a low acoustic transmission loss and a high water entry pressure. According to Patent Literature 1, important parameters on which to focus are the mass and thickness of the waterproof sound-transmitting membrane, not the air permeability of the membrane (i.e., air flow passing through the membrane). A reduction in both the mass and thickness of the waterproof sound-transmitting membrane leads to an increase in the acoustic energy transmitted by vibration of the membrane. Therefore, the acoustic transmission loss does not increase even if the air permeability is reduced to achieve a high water entry pressure. Patent Literature 1 discloses a waterproof sound-transmitting membrane having a thickness of 3 to 33 μm, a mass of 40 g/m2 or less, and an air permeability of 1 second or more in terms of Gurley number (i.e., about 1.57 cm3/cm2/s or less in terms of Frazier number).